This disclosure generally relates to systems and methods for the protection of imaging devices against light having a high radiant flux. As used herein, the term “high-radiant-flux light” means light having any one or more of the following measurable properties: high radiant intensity (W/sr), high irradiance (W/m2) or high radiance (W·sr−1·m−2). Such high-radiant-flux light may be coherent (e.g., laser light) or incoherent.
When operating a vehicle equipped with optical sensors, the general problem to be overcome is how to achieve high probability of mission success, at acceptably low cost, despite hazards. A hazard of increasing concern is high-radiant-flux light, which can damage optical sensors (hereinafter “image sensors”) needed to (a) operate a vehicle safely or (b) collect data, such as agricultural data or military surveillance data. This hazard usually arises from lasers aimed at a vehicle. However, it may also arise from arc welding equipment, exceptionally large or hot fires, a lightning bolt, or a nuclear blast.
Imaging devices, such as cameras and telescopes, are especially vulnerable to high-radiant-flux light. By design, such imaging devices use a lens or mirror to focus light onto an image sensor such as a focal plane array of opto-electrical elements that convert impinging light to electrical signals (hereinafter “array of photodetectors”) that acquire pixel data representing an image. This greatly increases the light intensity on the photodetectors corresponding to the location of the high-radiant-flux light source in the image. Thus, light that is harmless to a structural surface may have damaging radiant flux at the image sensor. The high-radiant-flux light can damage the image sensor by thermal shock, melting, or other mechanisms. A mechanism of particular concern is multi-photon absorption, which occurs when a femtosecond laser pulse strikes the surface of the image sensor. (As used herein, the term “femtosecond laser pulse” means a laser pulse having a pulse duration greater than or equal to one femtosecond but less than one picosecond.)
One approach to solving this problem is to shield or block electronic equipment from the high-radiant-flux light. A light-triggered shutter suffices for the weakest threats, such as accidental exposure to lasers used in a light show. Such a solution is insufficient for the more intense light encountered in military situations due to reaction time delays in such a system. An optical filter tuned to the specific laser wavelength works when the threat wavelength is known, but is ineffective against lasers with variable or unknown wavelength.
It would be advantageous to equip imaging devices with protection systems that can block even short, high-intensity laser pulses (e.g. femtosecond laser pulses) before the pulses damage the image sensor, and to do so across a wide range of wavelengths.